JP2021112689A - Operational method of reverse osmosis membrane - Google Patents

Abstract

To provide an operational method for reverse osmosis membranes capable of inhibiting a precipitation of inorganic substances in a treated water while also inhibiting a precipitation of organic substances and preventing blockage of a reverse osmosis membrane.SOLUTION: The present invention provides an operational method for reverse osmosis membranes in which reverse osmosis membranes separate raw water into permeate water and concentrated water. A pH of the treated water supplied to reverse osmosis membrane modules 18, 18, 18 is 5.8 to 7.8, and after treating the treated water with the reverse osmosis membranes of the reverse osmosis membrane modules 18, 18, 18, the reverse osmosis membranes of the reverse osmosis membrane modules 18, 18, 18 are periodically relieved of pressure.SELECTED DRAWING: Figure 1

Description

The present invention relates to a method of operating a reverse osmosis membrane.

A water treatment method for separating raw water into permeated water and concentrated water with a reverse osmosis membrane is known. FIG. 4 is a schematic schematic diagram for explaining an example of a conventional water treatment method using a reverse osmosis membrane. In the method using the treatment system 101 shown in FIG. 4, first, well water flows from the well 111 as raw water through the flow path 121 and is supplied to the water tank 112 to be treated. Chlorine is added to the water tank 112 to be treated by the chlorine adding means 130 and sulfuric acid is added by the sulfuric acid adding means 131, and raw water, chlorine and sulfuric acid are mixed to become water to be treated.
Next, the water to be treated flows through the flow path 122, polyaluminum chloride is added by the adding means 132, and the liquid is sent to the sand filtration tower 141 by the pump 114. In the sand filtration tower 141, aggregates of organic matter, iron, and manganese in the water to be treated are removed.
After that, the water to be treated is passed through the activated carbon 142, chlorine is added by the chlorine adding means 130, and then the water is supplied to the membrane filtration device 143 provided with the ultrafiltration membrane. In the filtration device 143, protozoans such as Cryptosporidium and various germs in the water to be treated are removed by the ultrafiltration membrane.

Next, the permeated water of the filtration device 143 flows through the flow path 127 and is stored in the intermediate treatment tank 144. A part of the permeated water in the intermediate treatment tank 144 is sent by the pump 114, flows through the flow path 127, and is stored as it is in the treatment water tank 119. On the other hand, the balance of the permeated water in the intermediate treatment tank 144 is sent by the pump 114, flows through the flow path 128, is boosted by the booster pump 116, and then is supplied to the reverse osmosis membrane filtration device 117 provided with the reverse osmosis membrane. .. In the reverse osmosis membrane filtration device 117, calcium and magnesium in the water to be treated are removed by the reverse osmosis membrane.
After that, the permeated water of the reverse osmosis membrane filtration device 117 flows through the flow path 124 and is supplied to the treated water tank 119. On the other hand, the concentrated water of the reverse osmosis membrane filtration device 117 is collected in the flow path 123 and discharged to the outside of the treatment system 101.
As described above, in the conventional method using the treatment system 101, treatment is performed for improving the water quality of each item of total organic carbon (TOC), iron and manganese, protozoans and germs, and hardness. Therefore, there is a problem that the apparatus configuration and the treatment flow become complicated and the cost of water quality treatment becomes high.

Therefore, as in the example shown in FIG. 5, the use of a treatment method in which all the treatments for improving the water quality of each item of TOC, iron and manganese, protozoans and germs, and hardness are collectively performed by the reverse osmosis membrane filtration device 117. Is being considered.
In the method using the treatment system 102 shown in FIG. 5, first, raw water flows from the well 111 through the flow path 121 and is supplied to the water tank 112 to be treated. Sulfuric acid is added into the water tank 112 to be treated by the sulfuric acid adding means 131, and the sulfuric acid and raw water are mixed to become water to be treated.
Next, the water to be treated is sent by the pump 114, flows through the flow path 122, is passed through the pre-filter 115 and the booster pump 116, and then is supplied to the reverse osmosis membrane filtration device 117. In the treatment system 102, the reverse osmosis membrane filtration device 117 removes organic substances in the water to be treated, inorganic substances such as calcium, silica, iron and manganese, protozoa and germs, calcium and magnesium all at once.
After that, the permeated water of the reverse osmosis membrane filtration device 117 flows through the flow path 124, sulfuric acid and chlorine are added, respectively, and the hardness is adjusted by the mineralizer 135. After that, the permeated water is stored in the treated water tank 119, and chlorine is added by the chlorine adding means 130 as needed.

As described above, according to the method using the treatment system 102, the water quality of almost all items can be improved by the reverse osmosis membrane filtration device 117, so that the device and the treatment flow can be simplified, the cost can be reduced, and the treatment system can be saved. Space can be achieved.
However, on the other hand, when the treatment for improving the water quality of a plurality of items is collectively performed by the reverse osmosis membrane filtration device 117 as in the method using the treatment system 102, there is a problem that the reverse osmosis membrane is frequently blocked. Occurs.
Therefore, it has been proposed to perform water treatment so that the pH of the water to be treated of the reverse osmosis membrane is 7.0 or less (for example, Patent Document 1).

JP-A-2019-107592

FIG. 6 is a schematic schematic diagram for explaining an example of a method of water treatment so that the pH of the water to be treated of the reverse osmosis membrane is 7.0 or less. In the treatment system 103 shown in FIG. 6, the concentrated water of the reverse osmosis membrane filtration device 117 is collected in the flow path 123, and a part of the concentrated water in the flow path 123 flows through the flow path 125 and is returned to the flow path 122. It is resupplied to the reverse osmosis membrane filtration device 117 as water to be treated.
In the method using the treatment system 103, the pH of the treated water supplied to the primary side of the reverse osmosis membrane of the reverse osmosis membrane filtration device 117 is 7.0 or less, as in the method disclosed in Patent Document 1. The pH of the water to be treated is adjusted in the water tank 112 to be treated by the sulfuric acid adding means 131. Therefore, it is possible to suppress the precipitation of inorganic substances in the water to be treated and reduce the deposition on the surface of the reverse osmosis membrane.
However, when water treatment is performed so that the pH of the water to be treated of the reverse osmosis membrane is 7.0 or less, organic substances are likely to precipitate from the water to be treated, and the surface of the reverse osmosis membrane is blocked by the precipitated organic substances. There is a problem.
The present invention provides a method for operating a reverse osmosis membrane, which can suppress the precipitation of organic substances while suppressing the precipitation of inorganic substances in the water to be treated, and can prevent the reverse osmosis membrane from being blocked.

The present invention has the following aspects.
[1] A method of operating a reverse osmosis membrane in which raw water is separated into permeated water and concentrated water by a reverse osmosis membrane, wherein the pH of the water to be treated supplied to the reverse osmosis membrane is 5.8 to 7. 8. A method of operating a reverse osmosis membrane, which comprises treating the water to be treated with the reverse osmosis membrane and then periodically releasing the pressure of the reverse osmosis membrane.
[2] The method for operating a reverse osmosis membrane according to [1], wherein when the pressure of the reverse osmosis membrane is released, the surface of the reverse osmosis membrane is washed by performing flushing.
[3] The method for operating a reverse osmosis membrane according to [2], which uses permeated water or washing water containing pure water at the time of flushing.
[4] The method for operating a reverse osmosis membrane according to any one of [1] to [3], wherein the raw water is directly treated with the reverse osmosis membrane.
[5] The method for operating a reverse osmosis membrane according to any one of [1] to [4], wherein the raw water further contains at least one or more inorganic substances selected from the group consisting of silica, calcium, magnesium, iron and manganese.
[6] The method for operating a reverse osmosis membrane according to any one of [1] to [5], wherein the M alkalinity of the raw water is 100 mg / L or more.
[7] The method for operating a reverse osmosis membrane according to any one of [1] to [6], wherein the total organic carbon of the water to be treated is 3.0 mg / L or more.

According to the operation method of the reverse osmosis membrane of the present invention, the precipitation of organic substances can be suppressed while suppressing the precipitation of inorganic substances in the water to be treated, and the blockage of the reverse osmosis membrane can be prevented.

It is a schematic schematic diagram of the processing system used for the operation method of the reverse osmosis membrane which concerns on one Embodiment. It is a schematic diagram which shows the structure of the flat membrane test apparatus used in an Example. FIG. 3 is a schematic cross-sectional view of a closed container and a flat membrane cell included in the flat membrane test apparatus of FIG. It is a schematic schematic diagram of the treatment system used in the conventional water treatment method. It is a schematic schematic diagram of the treatment system used in the conventional water treatment method. It is a schematic schematic diagram of the treatment system used in the conventional water treatment method.

“Total organic carbon” is, for example, JIS K 0101 20. It can be measured according to organic carbon (TOC).
“M alkalinity” is, for example, JIS K 0101 13. It can be measured according to the acid consumption.
"~" Indicating a numerical range means that the numerical values described before and after the numerical range are included as the lower limit value and the upper limit value.

Hereinafter, the method of operating the reverse osmosis membrane of the present invention will be described with reference to examples of embodiments. However, the present invention is not limited to the following embodiments.
FIG. 1 is a schematic schematic diagram of a processing system 1 used in a method of operating a reverse osmosis membrane according to an embodiment of the present invention. The treatment system 1 includes a well 11, a water tank 12 to be treated, an acid addition means 31, a reverse osmosis membrane filtration device 17, and a treatment water tank 19.

The water tank 12 to be treated is a tank for storing water to be treated. The raw water flow path 21, the water flow path 22 to be treated, and the circulation flow path 25 are connected to the water tank 12 to be treated.
The raw water flow path 21 is a flow path for supplying raw water (well water) from the well 11 to the water tank 12 to be treated. The water flow path 22 to be treated is a flow path for supplying water to be treated from the water tank 12 to the reverse osmosis membrane filtration device 17. The circulation flow path 25 is a flow path for supplying the concentrated water of the reverse osmosis membrane filtration device 17 to the water tank 12 to be treated. Therefore, the water to be treated in the water tank 12 to be treated includes the concentrated water of the reverse osmosis membrane filtration device 17 and the raw water (well water).

The acid addition means 31 adds an acid component to the water to be treated in the water tank 12 to be treated. The acid component is not particularly limited. For example, hydrochloric acid, sulfuric acid and the like can be mentioned.
The acid addition means 31 may be in the form of directly adding the acid component to the water to be treated in the liquid phase portion in the water tank 12 to be treated, or may be in the form of adding the acid component via the gas phase portion in the water tank 12 to be treated. ..

The pump 14, the pre-filter 15, and the booster pump 16 are provided in the water flow path 22 to be treated in this order from the water tank 12 side to be treated.
The pump 14 is for feeding and supplying the water to be treated to the reverse osmosis membrane filtration device 17.
The pre-filter 15 removes large-sized impurities such as sand, silt, and clay, and protects the reverse osmosis membrane filtration device 17 from clogging, scratching, and the like.
The booster pump 16 applies the pressure required for filtering the water to be treated by the reverse osmosis membrane filtration device 17.

The reverse osmosis membrane filtration device 17 includes reverse osmosis membrane modules 18, 18, 18. The reverse osmosis membrane modules 18, 18, 18 each have a reverse osmosis membrane inside. The reverse osmosis membrane filtration device 17 is a device that reverse osmosis the water to be treated into the reverse osmosis membranes in the reverse osmosis membrane modules 18, 18 and 18 to separate the water to be treated into permeated water and concentrated water.
Each end of the branched water flow path 22 to be treated is connected to each reverse osmosis membrane module 18. Therefore, the water to be treated in the water flow path 22 to be treated is supplied to the primary side of the reverse osmosis membrane of each reverse osmosis membrane module 18.

Further, each of the reverse osmosis membrane modules 18 is connected to the branched ends of the concentrated water flow path 23 and the permeated water flow path 24, respectively.
The concentrated water flow path 23 is a flow path for collecting the concentrated water separated by the reverse osmosis membranes in the reverse osmosis membrane modules 18, 18, 18. The concentrated water flow path 23 is connected to the circulation flow path 25 and the drainage flow path 26 at the connection point D. The circulation flow path 25 is a flow path for supplying a part of the concentrated water from the concentrated water flow path 23 to the water tank 12 to be treated. The drainage flow path 26 is a flow path for discharging the remaining portion of the concentrated water to the outside of the treatment system 1.
The permeated water flow path 24 is connected to the treated water tank 19. The permeated water flow path 24 is a flow path for supplying the permeated water separated by the reverse osmosis membrane filtration device 17 to the treated water tank 19.
The treated water tank 19 is a tank that stores the permeated water separated by the reverse osmosis membrane filtration device 17 as treated water.

Next, a method of operating the reverse osmosis membrane according to the embodiment of the present invention will be described.
The method of operating the reverse osmosis membrane according to the present embodiment is a method of treating raw water by separating it into permeated water and concentrated water with a reverse osmosis membrane.
Examples of raw water include groundwater such as well water and underground water; river water; and lake water. However, raw water is not limited to these examples.

In this embodiment, the raw water contains soluble organic matter. The organic matter in the raw water is not particularly limited, and examples thereof include humic acid. Examples of the humic acid in the raw water include humic acid and fulvic acid.
The total organic carbon in the raw water is preferably 3.0 mg / L or more, more preferably 4.0 mg / L or more, from the viewpoint of suppressing the amount of the acid component used, suppressing the membrane blockage of the reverse osmosis membrane, and the like. 0 mg / L or more is particularly preferable. In the present embodiment, since the lower limit of the pH of the water to be treated is adjusted to a predetermined value or more as described later, clogging of the reverse osmosis membrane can be prevented even in raw water having total organic carbon of the lower limit or more. As a result, even when the total organic carbon in the raw water is high, the permeation ability is less likely to decrease due to clogging of the reverse osmosis membrane, and the permeated water can be obtained while maintaining the treatment efficiency.
The upper limit of total organic carbon in raw water is not particularly limited. For example, the total organic carbon in the raw water is preferably 100 mg / L or less, more preferably 50 mg / L or less, from the viewpoint of treatment efficiency.

The content of the organic compound having a molecular weight of 10,000 or more in the raw water may be 0.001 to 1 mg / L, 0.005 to 0.5 mg / L, or 0.01 to 0.1 mg / L. When the content of the organic compound having a molecular weight of 10,000 or more in the raw water is not more than the upper limit value, precipitation of the organic substance is unlikely to occur, and the clogging of the reverse osmosis membrane by the precipitated organic substance can be further suppressed. Even if the content of the organic compound having a molecular weight of 10,000 or more in the raw water is equal to or higher than the lower limit, in the present embodiment, the lower limit of the pH of the water to be treated is adjusted to a predetermined value or higher as described later. It is considered that organic substances are less likely to precipitate, blockage of the reverse osmosis membrane can be prevented, and permeated water can be obtained while maintaining treatment efficiency.

In addition to organic matter, the raw water may further contain at least one or more ionic inorganic matter selected from the group consisting of silica, calcium, magnesium, iron and manganese. When the raw water further contains these inorganic substances, in the present embodiment, since the upper limit of the pH of the water to be treated is adjusted to a predetermined value or less as described later, the inorganic substances are less likely to precipitate and the reverse osmosis membrane is prevented from being blocked. It is considered that permeated water can be obtained while maintaining the treatment efficiency.

The M alkalinity of the raw water is preferably 100 mg / L or more, more preferably 200 mg / L or more, and particularly preferably 300 mg / L or more, from the viewpoints of suppressing the amount of acid components used, suppressing membrane blockage of the reverse osmosis membrane, and the like. .. Even if the M alkalinity of the raw water is equal to or higher than the above lower limit value, in the present embodiment, the lower limit value of the pH of the water to be treated is adjusted to a predetermined value or higher, so that the amount of the acid component used is unlikely to increase, and the water treatment Cost reduction can be achieved.
The upper limit of the M alkalinity of raw water is not particularly limited. For example, from the viewpoint of treatment efficiency, the M alkalinity of the raw water is preferably 1000 mg / L or less, more preferably 700 mg / L or less.

In the operation method of the reverse osmosis membrane according to the present embodiment, the raw water is separated into permeated water and concentrated water by the reverse osmosis membrane. As shown in FIG. 1, first, the raw water flows from the well 11 through the raw water flow path 21 and is supplied to the water tank 12 to be treated. In the water tank 12 to be treated, the acid component is added by the acid adding means 31, and the acid component and the raw water are mixed. In addition, a dispersant may be further added to the raw water for the purpose of dispersing the organic matter in the raw water. The addition of the dispersant can be expected to improve the cleaning effect on the surface of the reverse osmosis membrane during flushing, which will be described later.
Next, the water to be treated is sent by the pump 14 and flows through the water flow path 22 to be treated. After that, the water to be treated is supplied to the reverse osmosis membrane of the reverse osmosis membrane module 18 in the reverse osmosis membrane filtration device 17 via the prefilter 15 and the booster pump 16 in this order, and is treated by the reverse osmosis membrane.

As described above, in the method of operating the reverse osmosis membrane according to the present embodiment, it is preferable to directly treat the raw water (well water) with the reverse osmosis membrane of the reverse osmosis membrane module 18. "Directly treating raw water with a reverse osmosis membrane" means supplying the raw water to the reverse osmosis membrane without pretreating the raw water such as groundwater and well water. Here, the pretreatment includes, for example, a treatment performed for the purpose of reducing total organic carbon, removing iron and manganese, removing protozoans and germs, and the like.
In the conventional water treatment method using a reverse osmosis membrane, it has been recommended that the total amount of impurities and the like in the water to be treated supplied to the reverse osmosis membrane be reduced to a certain amount or less and then supplied to the reverse osmosis membrane. This is to prevent the reverse osmosis membrane from being damaged due to a decrease in permeability due to clogging of the reverse osmosis membrane and an increase in pressure difference for maintaining the permeability.
On the other hand, in the operation method of the reverse osmosis membrane according to the present embodiment, the pH of the water to be treated of the reverse osmosis membrane is controlled within a predetermined range as described later, so that precipitates are generated from the water to be treated. Is suppressed. Therefore, even if groundwater (raw water) such as well water is directly supplied to the reverse osmosis membrane as water to be treated, the reverse osmosis membrane can continuously obtain permeated water. By treating the raw water directly with the reverse osmosis membrane of the reverse osmosis membrane module 18, the water quality of each item of total organic carbon (TOC), iron and manganese, protozoa and germs, and hardness can be improved by the reverse osmosis membrane. .. Therefore, the apparatus and the treatment flow can be simplified, the cost of water treatment can be reduced, and the space of the treatment system 1 can be saved.

Next, the water to be treated supplied to each reverse osmosis membrane module 18 is separated into permeated water and concentrated water by the reverse osmosis membrane of each reverse osmosis membrane module 18. After that, each permeated water of each reverse osmosis membrane module 18 flows through the permeated water flow path 24 and is stored as treated water in the treated water tank 19.
On the other hand, each concentrated water of each reverse osmosis membrane module 18 is collected in the concentrated water flow path 23. A part of the concentrated water in the concentrated water flow path 23 then flows through the circulation flow path 25 and is supplied to the water tank 12 to be treated again. Therefore, a part of the concentrated water is mixed with the raw water from the well 11 in the water tank 12 to be treated, and is re-supplied to the reverse osmosis membrane filtration device 17 as the water to be treated. Further, the remaining portion of the concentrated water in the concentrated water flow path 23 flows through the drainage flow path 26 and is discharged to the outside of the treatment system 1.

In the method of operating the reverse osmosis membrane according to the present embodiment, the pH of the water to be treated supplied to each reverse osmosis membrane of each reverse osmosis membrane module 18 is adjusted to be 5.8 to 7.8.
The pH of the water to be treated of each reverse osmosis membrane module 18 is in the range of 5.8 to 7.8, preferably in the range of 6.3 to 7.5, and in the range of 6.5 to 7.3. Is more preferable, and the range of 6.7 to 7.2 is particularly preferable.
When the pH of the water to be treated is equal to or higher than the lower limit, organic substances are less likely to precipitate on the surface of the reverse osmosis membrane on the primary side of each reverse osmosis membrane module 18, and clogging of the reverse osmosis membrane can be prevented. In addition, as compared with the conventional operation method in which the lower limit of the pH of the water to be treated is not particularly limited, the amount of the acid component used for lowering the pH is reduced, and the cost can be reduced. Further, when the pH of the water to be treated is equal to or lower than the upper limit value, oxides of metals such as iron and manganese are less likely to precipitate on the surface of the primary side of the reverse osmosis membrane of each reverse osmosis membrane module 18, and reverse osmosis Blockage of the membrane can be prevented.
The pH of the water to be treated can be adjusted, for example, by adding an acid component by the acid adding means 31.

The total organic carbon in the water to be treated supplied to each reverse osmosis membrane module 18 may be 3.0 mg / L or more, 5.0 mg / L or more, or 10.0 mg / L or more. In the present embodiment, since the lower limit of the pH of the water to be treated is adjusted to be equal to or higher than a predetermined value, clogging of the reverse osmosis membrane is prevented even if the total organic carbon in the water to be treated is equal to or higher than the lower limit. Therefore, the permeation ability is less likely to decrease due to clogging of the reverse osmosis membrane, and permeated water can be obtained while maintaining the treatment efficiency.
The upper limit of total organic carbon in the water to be treated is not particularly limited. For example, the total organic carbon in the water to be treated may be 100 mg / L or less, or 500 mg / L or less.

In the method of operating a reverse osmosis membrane according to the present embodiment, the water to be treated is filtered and treated by the reverse osmosis membrane of each reverse osmosis membrane module 18 for a predetermined time.
When filtering the water to be treated with the reverse osmosis membrane, each reverse osmosis membrane module 18 is filled with the water to be treated, the water pressure in each reverse osmosis membrane module 18 becomes high, and pressure is applied to the reverse osmosis membrane. To go. After that, when the water pressure applied to each reverse osmosis membrane becomes higher than the osmotic pressure, a reverse osmosis phenomenon occurs in the reverse osmosis membrane, the solute concentration in the water to be treated becomes locally high on the primary side of each reverse osmosis membrane, and the permeated water becomes can get. As a result, concentrated water is generated on the primary side of each reverse osmosis membrane, and each concentrated water is separated from each permeated water on the secondary side of each reverse osmosis membrane. In this way, the water to be treated can be filtered and treated.

The time for filtering the water to be treated by the reverse osmosis membrane is not particularly limited, and can be appropriately set in consideration of treatment efficiency, for example. The time for filtering the water to be treated by the reverse osmosis membrane may be, for example, 5 to 1500 minutes or 15 to 180 minutes. When the time for filtering the water to be treated is equal to or longer than the above lower limit value, the amount of permeated water obtained increases, and the treatment efficiency tends to be maintained sufficiently high in practice. When the time for filtering the water to be treated is equal to or less than the upper limit value, it becomes easier to prevent the reverse osmosis membrane from being blocked.

In the method of operating a reverse osmosis membrane according to the present embodiment, the risk of blockage of the reverse osmosis membrane increases after the water to be treated is filtered and treated with the reverse osmosis membrane for a predetermined time.
Therefore, in the method of operating a reverse osmosis membrane according to the present embodiment, after treating the water to be treated for a predetermined time, the pressure of the reverse osmosis membrane of each reverse osmosis membrane module 18 is released periodically. By releasing the pressure of the reverse osmosis membrane on a regular basis, the solute on the surface of the reverse osmosis membrane diffuses, and the state of concentration polarization is eliminated. Therefore, the concentration polarization on the primary side of the reverse osmosis membrane can be periodically eliminated, and the precipitation of inorganic substances and organic substances on the surface of the primary side of the reverse osmosis membrane can be prevented.
Here, "releasing the pressure of the reverse osmosis membrane periodically" means that after filtering the water to be treated for a predetermined period, the reverse osmosis membrane is released from the water pressure of the water to be treated in each reverse osmosis membrane module 18. Means that. As a general rule, the "predetermined period" here means a certain period. However, the "predetermined period" does not necessarily have to be a strictly constant period, and may be a slight period (for example, a difference of about 1 to 10 minutes) as long as the effect of the present invention can be obtained. Long and short are acceptable.

When releasing the pressure of the reverse osmosis membrane, the pressure may be released on the secondary side (permeated water side) of the reverse osmosis membrane. By releasing the pressure on the secondary side (permeated water side) of the reverse osmosis membrane, the water content on the secondary side of the reverse osmosis membrane moves to the primary side (water side to be treated) of the reverse osmosis membrane by forward osmosis. As a result, the solute concentration on the primary side of the reverse osmosis membrane can be reduced periodically, and the precipitation of inorganic substances and organic substances on the surface of the primary side of the reverse osmosis membrane can be reliably and prevented.

The time for releasing the pressure of the reverse osmosis membrane is not particularly limited, and can be appropriately set in consideration of, for example, processing efficiency. The time for releasing the pressure of the reverse osmosis membrane may be, for example, 10 to 600 seconds or 30 to 120 seconds. When the time for releasing the pressure of the reverse osmosis membrane is equal to or longer than the above lower limit value, it is easy to prevent the precipitation of inorganic substances and organic substances on the surface of the primary side of the reverse osmosis membrane. When the time for releasing the pressure of the reverse osmosis membrane is not more than the above upper limit value, the treatment efficiency of the permeated water tends to be maintained sufficiently high in practical use.

The specific method for releasing the pressure of the reverse osmosis membrane is a method that can release the reverse osmosis membrane from the water pressure of the water to be treated in each reverse osmosis membrane module 18 to eliminate the concentration polarization on the surface of the reverse osmosis membrane. For example, there is no particular limitation. For example, the following methods can be mentioned, but the specific mode of pressure release is not limited to the following examples.
For example, the reverse osmosis membrane filtration device 17 has an extraction pipe (not shown) for extracting the water to be treated supplied into the reverse osmosis membrane module 18, and an extraction valve (not shown) provided in the extraction pipe. In this case, the pressure of the reverse osmosis membrane can be released by loosening the extraction valve and reducing the pressure on the primary side of the reverse osmosis membrane.
Alternatively, the pressure of the reverse osmosis membrane may be released by stopping the booster pump 16 to reduce the pressure on the primary side of the reverse osmosis membrane.

In the operation method of the reverse osmosis membrane according to the present embodiment, even if a component such as an organic substance is precipitated from the water to be treated, or even if the precipitate is deposited on the surface of the primary side of the reverse osmosis membrane, the pressure is released. At that time, further flushing can be performed to wash away the deposits on the surface of the reverse osmosis membrane. Therefore, blockage of the reverse osmosis membrane can be prevented more effectively.

The specific method for performing flushing is not particularly limited. For example, the following methods can be mentioned, but the specific mode of flushing is not limited to the following examples.
For example, the flow rate of the water to be treated, which is sent into each reverse osmosis membrane module 18 through the water flow path 22 to be treated by the pump 14 and the booster pump 16, is made larger than the flow rate when the water to be treated is filtered. Can be flushed with. As a result, more water to be treated than the flow rate when filtering the water to be treated flows on the surface of the reverse osmosis membrane as wash water, and the precipitates accumulated on the surface of the reverse osmosis membrane can be washed away, so that the reverse osmosis membrane can be washed away. The effect of preventing blockage is further enhanced.
In addition, a back pressure valve is installed in the concentrated water flow path 23, the opening degree of the back pressure valve is increased, and more water to be treated flows to the surface of the reverse osmosis membrane than the flow rate when filtering the water to be treated. Then, flushing may be carried out.

At the time of flushing, in addition to raw water and concentrated water, washing water containing permeated water or pure water in the water to be treated may be used. When wash water containing permeable water or pure water is used, the reverse osmosis membrane can be washed with cleaner wash water, and deposits containing organic substances and the like deposited on the surface of the reverse osmosis membrane can be removed more effectively. The effect of preventing blockage is further enhanced.

The flow velocity of the water to be treated during flushing is preferably 0.05 m / s or more, more preferably 0.2 m / s or more, and even more preferably 0.5 m / s or more. When the flow velocity of the water to be treated during flushing is equal to or higher than the lower limit value, further improvement of the cleaning effect by flushing can be expected. Here, the flow velocity of the water to be treated at the time of flushing is measured as the flow velocity of the water to be treated on the surface of the reverse osmosis membrane of the reverse osmosis membrane module 18.

Flushing is performed when the pressure of the reverse osmosis membrane is released periodically, but flushing itself may be performed periodically after a certain period of time each time the pressure of the reverse osmosis membrane is released. In addition, it may be done irregularly. However, considering the cleaning effect of the reverse osmosis membrane and the recovery effect of the separating ability by flushing, it is considered preferable to perform flushing on a regular basis in the same manner as the pressure release.

(Action effect)
In the operation method of the reverse osmosis membrane according to the present embodiment described above, in order to adjust the pH of the water to be treated supplied to the reverse osmosis membrane module 18 to be 5.8 or more, the primary side of the reverse osmosis membrane is adjusted. Organic substances are less likely to precipitate on the surface, and blockage of the reverse osmosis membrane by organic substances can be suppressed.
Further, in the operation method of the reverse osmosis membrane according to the present embodiment, in order to adjust the pH of the water to be treated supplied to the reverse osmosis membrane module 18 to 7.8 or less, the reverse osmosis membrane of the reverse osmosis membrane module 18 is adjusted. Inorganic substances are less likely to precipitate on the surface on the primary side, and blockage of the reverse osmosis membrane due to metal oxide can be suppressed.
In addition, in the method of operating a reverse osmosis membrane according to the present embodiment, the pressure of the reverse osmosis membrane is periodically released after the water to be treated is treated with the reverse osmosis membrane. Therefore, the state of concentration polarization on the surface of the reverse osmosis membrane after filtering the water to be treated can be periodically eliminated. As a result, it is possible to suppress the precipitation of inorganic substances and organic substances on the surface of the primary side of the reverse osmosis film, and prevent the reverse osmosis film from being clogged by the inorganic substances and organic substances.
Therefore, according to the operation method of the reverse osmosis membrane according to the present embodiment, the precipitation of organic substances can be suppressed while suppressing the precipitation of inorganic substances in the water to be treated, and the blockage of the reverse osmosis membrane can be prevented.
Furthermore, when the pressure is released on the secondary side of the reverse osmosis membrane, the permeated water can be periodically moved from the secondary side (permeated water side) to the primary side (water to be treated) by forward permeation. be. Therefore, in this case, the solute concentration of the organic substance on the primary side of the reverse osmosis membrane can be periodically reduced. As a result, it is possible to reliably prevent the reverse osmosis membrane from being clogged by inorganic substances and organic substances on the surface of the primary side of the reverse osmosis membrane.

Further, in the method of operating the reverse osmosis membrane according to the present embodiment, the pH of the water to be treated supplied to the reverse osmosis membrane module 18 is adjusted to be 5.8 or more. Therefore, the amount of the acid component used is reduced as compared with the case where the pH is lowered to less than 5.8, and the effect that the cost of water treatment can be reduced can be obtained.

In the conventional operation method in which the reverse osmosis membrane is washed only by flushing, the washing water after flushing may contain a large amount of deposits. The flushing water containing such a large amount of deposits cannot be reused as the water to be treated for filtering the reverse osmosis membrane, and may have to be discarded as it is. As described above, when the water to be treated is used for flushing and then discarded, it may cause a decrease in the recovery efficiency of the entire water treatment system that recovers the treated water from the water to be treated.
On the other hand, in the operation method of the reverse osmosis membrane according to the present embodiment, the blockage of the reverse osmosis membrane can be prevented by periodically releasing the pressure of the reverse osmosis membrane. Can be reduced. In addition, the amount of deposits on the surface of the reverse osmosis membrane is reduced, and even if flushing is performed, the amount of deposits mixed in the washing water after flushing is considered to be relatively small. Therefore, the wash water after flushing can be sufficiently reused as the water to be treated for filtration of the reverse osmosis membrane, and even when the water to be treated is used for flushing, the recovery efficiency of the treated water does not decrease. The treatment efficiency of the water treatment system is improved.
As described above, according to the operation method of the reverse osmosis membrane according to the present embodiment, the amount of treated water that can be recovered from the same amount of water to be treated and the recovery efficiency can be expected to be improved, and the cost of water treatment can be reduced. Can be done.

Hereinafter, the present invention will be described in more detail with reference to Examples. However, the present invention is not limited to the examples, and various modifications can be made without departing from the spirit of the present invention.

Well water was commonly used as raw water in this example and comparative example. The measured values of each water quality item of well water before concentration are described below.
pH: 8.3
Total organic carbon: 2.0 mg / L
Iron: 0.03 mg / L
Manganese: less than 0.005 mg / L Calcium: 16 mg / L
Magnesium: 5 mg / L
Silica: 52 mg / L
M alkalinity: 233 mg / L

In this example and comparative example, the flat membrane test apparatus 40 shown in FIG. 2 was used. The flat membrane test apparatus 40 includes a container 41, a flat membrane cell 42, a supply pipe 43, a permeated water pipe 44, a concentrated water pipe 45, a pump 46, a pressure gauge 47, a back pressure valve 48, a stirrer 49, and a stirrer 50.
FIG. 3 is a schematic cross-sectional view showing the container 41 and the flat membrane cell 42 of the flat membrane test apparatus 40 of FIG. A flat membrane cell 42 is fixed to the container 41, and the container 41 has a first case 41a on the lower side of the flat membrane cell 42 and a second case 41b on the upper side of the flat membrane cell 42. A flat membrane cell 42 is fixed between the first case 41a and the second case 41b via an O-ring 51. The space surrounded by the first case 41a under the flat membrane cell 42 becomes the raw water chamber A. Further, the space surrounded by the second case 41b on the upper side of the flat membrane cell 42 becomes the permeation water chamber B. The flat membrane cell 42 has a reverse osmosis membrane 52 and a porous support plate 53 that supports the surface of the reverse osmosis membrane 52 on the water permeation chamber B side (permeation water side).

In the flat membrane test apparatus 40, as shown in FIGS. 2 and 3, the water to be treated flows through the supply pipe 43, is boosted by the pump 46, and is supplied to the lower raw water chamber A in the closed container 41. The water to be treated supplied into the raw water chamber A is agitated by the stirrer 50 rotated by the stirrer 49, filled in the raw water chamber A and pressurized, and treated by the reverse osmosis membrane 52. The permeated water of the reverse osmosis membrane 52 flows through the permeated water pipe 44 through the permeated water chamber B above the flat membrane cell 42. The concentrated water of the reverse osmosis membrane 52 flows through the concentrated water pipe 43. When treating the water to be treated by the reverse osmosis membrane 52, the pressure in the raw water chamber A can be adjusted by the back pressure valve 48 provided in the concentrated water pipe 45, and is measured by the pressure gauge 47. As the reverse osmosis membrane 52 in Examples and Comparative Examples, “ESPA2 (membrane area: 8 cm ² )” manufactured by Nitto Denko was used.

<Example 1>
First, sulfuric acid was added to adjust the pH of the well water to 7.0. Then, 5 L of well water whose pH was adjusted to 7.0 was concentrated 3.3 times with the flat membrane test apparatus 40. At the time of concentration, the opening degree of the back pressure valve 48 is adjusted to set the supply pressure of the well water to 0.5 MPa, and the concentrated water of the reverse osmosis membrane 52 is resupplied to the supply pipe 43 to be circulated, while the reverse osmosis membrane 52 is circulated. Well water was concentrated by discharging the permeated water from the system to the outside of the system. Then, sulfuric acid was added to the concentrated well water to adjust the pH to 7.0 again to prepare the water to be treated.
Then, the water to be treated was treated with a reverse osmosis membrane for 59 minutes and separated into permeated water and concentrated water. When treating the water to be treated, the opening degree of the back pressure valve 48 is adjusted so that the water pressure becomes 0.5 MPa, and the permeated water and the concentrated water are supplied again to the supply pipe 44 to supply the permeated water and the concentrated water. The mixture was circulated while being mixed, and the change over time in the permeation flow rate of the reverse osmosis membrane 52 was observed.
Then, the back pressure valve 48 was opened for 1 minute to release the pressure of the reverse osmosis membrane. The process of separating the water to be treated for 59 minutes and then releasing the pressure of the reverse osmosis membrane for 1 minute was defined as one cycle for a total of 60 minutes, and this step was performed for a total of 100 cycles.
The ratio of the flow rate of the permeated water after 100 cycles to the flow rate of the permeated water 0 minutes after the start of separation of the water to be treated was calculated as a relative flux. The results are shown in Table 1. Table 1 also shows the Langeria index (LSI) of the water to be treated whose pH was adjusted to 7.0 and the amount of sulfuric acid used to adjust the pH of the water to be treated.

<Example 2>
First, sulfuric acid was added to adjust the pH of the well water to 6.0. Then, 5 L of well water whose pH was adjusted to 6.0 was concentrated 3.3 times in the same manner as in Example 1 with the flat membrane test apparatus 40. Then, sulfuric acid was added to the concentrated well water to adjust the pH to 6.0 again to prepare the water to be treated.
Then, the water to be treated was treated with a reverse osmosis membrane for 60 minutes to separate it into permeated water and concentrated water. When treating the water to be treated, the opening degree of the back pressure valve 48 is adjusted so that the water pressure becomes 0.5 MPa, and the permeated water and the concentrated water are supplied again to the supply pipe 44 to supply the permeated water and the concentrated water. The mixture was circulated while being mixed, and the change over time in the permeation flow rate of the reverse osmosis membrane 52 was observed.
Then, for 0.5 minutes (30 seconds), the back pressure valve 48 was opened to release the pressure of the reverse osmosis membrane. The process of separating the water to be treated for 60 minutes and then releasing the pressure of the reverse osmosis membrane for 0.5 minutes (30 seconds) for a total of 60.5 minutes was defined as one cycle, and this process was performed for a total of 100 cycles. ..
The ratio of the flow rate of the permeated water after 100 cycles to the flow rate of the permeated water 0 minutes after the start of separation of the water to be treated was calculated as a relative flux. The results are shown in Table 1. Table 1 also shows the Langeria index (LSI) of the water to be treated whose pH was adjusted to 6.0 and the amount of sulfuric acid used to adjust the pH of the water to be treated.

<Comparative example 1>
The pH of the water to be treated is adjusted to 6.0, the water to be treated is treated with a reverse osmosis membrane, and the water to be treated is permeated with the reverse osmosis membrane for 100 hours without periodic pressure release and flushing. Treated water was obtained in the same manner as in Example 1 except that it was separated into water and concentrated water.

<Comparative example 2>
Treatment was carried out in the same manner as in Example 1 except that the raw water was treated with a reverse osmosis membrane while keeping the pH of the water to be treated at 8.3 without adding sulfuric acid as an acid component to the water to be treated in the water tank 12 to be treated. I got water.

<Comparative example 3>
The treated water was obtained in the same manner as in Example 1 except that the pH of the water to be treated was adjusted to 5.5 and the water to be treated was treated with a reverse osmosis membrane.

As shown in Table 1, the amounts of sulfuric acid used for adjusting the pH in Examples 1 and 2 were 57 mg / L and 171 mg / L, respectively. The relative fluxes after 100 hours were 0.92 and 0.97, respectively.

On the other hand, in Comparative Example 1 in which regular pressure release and flushing were not performed, the relative flux after 100 hours decreased to 0.87. It was considered that the cause of the decrease in the relative flux in Comparative Example 1 was that the organic matter deposited on the surface of the reverse osmosis membrane was deposited.

In Comparative Example 2 in which the water to be treated was treated with a reverse osmosis membrane while the pH of the water to be treated was 8.3 without adding sulfuric acid as an acid component to the water to be treated, the relative flux after 100 hours was 0.49. It had dropped to. The reason why the relative flux decreased in Comparative Example 2 is that the pH of the water to be treated exceeds the upper limit specified in the present invention, so that oxides of metals such as iron and manganese in the water to be treated are precipitated and precipitated. It was considered that this was because the minerals were deposited on the surface of the reverse osmosis membrane.

From the results of the examples and comparative examples described above, according to the operation method of the reverse osmosis membrane according to the present embodiment, the precipitation of organic substances can be suppressed while suppressing the precipitation of inorganic substances in the water to be treated, and the reverse osmosis membrane can be suppressed. It was thought that blockage could be prevented.
In addition, in Comparative Example 3, the amount of sulfuric acid used to adjust the pH of the water to be treated to 5.5 was 220 mg / L, which was increased as compared with the amount of sulfuric acid used in Examples 1 and 2. As described above, in Examples 1 and 2, the pH of the water to be treated could be adjusted within a range in which the clogging of the reverse osmosis membrane could be suppressed with a smaller amount of the acid component used. Therefore, according to the operation method of the reverse osmosis membrane according to the present embodiment, it is considered that the cost of water treatment can be reduced.

1 Water treatment system 11 Well 12 Processed water tank 14 Pump 15 Prefilter 16 Booster pump 17 Reverse osmosis membrane filtration device 18 Reverse osmosis membrane module 19 Treated water tank 21 Raw water flow path 22 Processed water flow path 23 Concentrated water flow path 24 Permeated water flow path 25 Circulation channel 26 Drainage channel 31 Acid addition means

Claims (7)

A method of operating a reverse osmosis membrane that separates raw water into permeated water and concentrated water using a reverse osmosis membrane.
The pH of the water to be treated supplied to the reverse osmosis membrane is 5.8 to 7.8, and the pH is 5.8 to 7.8.
A method for operating a reverse osmosis membrane, in which the pressure of the reverse osmosis membrane is periodically released after treating the water to be treated with the reverse osmosis membrane. The method for operating a reverse osmosis membrane according to claim 1, wherein the surface of the reverse osmosis membrane is washed by performing flushing when the pressure of the reverse osmosis membrane is released. The method for operating a reverse osmosis membrane according to claim 2, wherein wash water containing permeated water or pure water is used for flushing. The method for operating a reverse osmosis membrane according to any one of claims 1 to 3, wherein the raw water is directly treated with the reverse osmosis membrane. The method for operating a reverse osmosis membrane according to any one of claims 1 to 4, wherein the raw water further contains at least one or more inorganic substances selected from the group consisting of silica, calcium, magnesium, iron and manganese. The method for operating a reverse osmosis membrane according to any one of claims 1 to 5, wherein the M alkalinity of the raw water is 100 mg / L or more. The method for operating a reverse osmosis membrane according to any one of claims 1 to 6, wherein the total organic carbon of the water to be treated is 3.0 mg / L or more.

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